I am installing a Mustee preformed fiberglass shower in my basement. The drain piping was roughed-in when the house was built. In the concrete floor is a 4" dia. PVC pipe cut flush with the floor surface. Of course the shower drain pipe is 2". Do I install some sort of flange to make this connection? If yes, do I sink the flange below the floor surface to allow the shower base to sit level? Thanks for the answer.